This invention relates in general to plasma arc cutting torches and more particularly to an improved nozzle construction that provides enhanced cooling and a more reliable initial height sensing.
The present invention is an improvement over the cutting torches and height sensing method described in U.S. Pat. Nos. 3,641,308 and 4,203,022. The U.S. Pat. No. 3,641,308 patent describes a nozzle for a plasma arc cutting torch where a laminar jet of a cooling liquid, usually water, surrounds and constricts the plasma. The liquid jet is created by a pair of generally annular members which together form the nozzle of the torch. The inner member has a central passageway that directs an ionizable gas from an electrode to a workpiece located below the electrode. The outer member cooperates with the inner one to create an annular nozzle that emits the liquid jet. The effectiveness of this system is concentrating the arc depends, in part, on the water mass flow rate and flow velocity creating the constricting water jet.
The U.S. Pat. No. 4,203,022 patent describes a method and apparatus for sensing the height of the torch over a workpiece, particularly as the torch is lowered toward the workpiece to begin cutting. Height sensing is important since the outer annular member of the torch nozzle is typically a ceramic material that can be damaged through contact with the workpiece. The U.S. Pat. No. 4,203,022 patent teaches that a gas flow can be introduced through the gas system as the torch is lowered. The gas stream swirls and has a vortex pressure. When the torch is closely spaced from the workpiece, the vortex "attaches" to the workpiece resulting in an abrupt change in the vortex pressure. This change is sensed and gives the desired height information. One source of unreliability with this system arises out of residual water which is often present in the water ejection area of the nozzle, particularly when the nozzle is worn. Also, during the height sensing operation, if a droplet of residual water is ejected by the gas flow, the droplet can cause a pressure fluctuation which can be interpreted incorrectly as an indication that the torch is at the proper height over the workpiece.
More generally, the performance of a plasma arc cutting torch is directly related to the ability of the cutting system to cool the nozzle of the torch. The cooler the nozzle, the larger the current that the plasma can conduit. Also, a cool nozzle has an extended life since less scale and deposits form on the nozzle. In a water cooled torch of the general type shown in U.S. Pat. Nos. 3,641,308 and 4,203,022, heretofore it has not been possible to simply increase cooling by increasing the water flow because the laminar jet used to constrict the plasma is sensitive to flow parameters.
It is therefore a principal object of this invention to provide a plasma arc cutting torch that has its plasma arc concentrated by a jet of a cooling liquid while at the same time offering markedly increased cooling as compared to conventional torches of this type.
Another object is to provide a nozzle construction with an extended useful life.
A further object is to provide a nozzle construction which avoids unreliability in the initial height sensing due to the presence of residual water in the nozzle.
Yet another object is to provide an improved plasma arc cutting torch with the foregoing advantages which has a competitive cost of manufacture as compared to conventional torches of the same general type.
These and other objects and features will be more fully understood from the following detailed description which should be read in light of the accompanying drawings.